1. Field of the Invention
The present invention relates to an electric deionizing apparatus and a process for electric deionization using the apparatus. More particularly, the present invention relates to an electric deionizing apparatus which effectively removes silica and boron components, in particular, silica components, from material water (water to be deionized) and provides deionized water having a high resistivity and a process for deionization using the apparatus.
2. Description of the Related Art
Water has been used for industrial, household and agricultural uses. As water is more extensively used in these areas, the importance of water is increasing. Specific types of water such as pure water and ultrapure water are being required in chemical industries and novel industries of advanced technology. On the other hand, utilization of water of lower qualities such as sewage, regenerated industrial waste water and rain water has been advancing in order to save water resources. For example, water of lower qualities is used for miscellaneous applications in buildings such as water for flushing toilets, cooling and cleaning.
As the technology for deionizing industrial water, sewage and industrial waste water to produce water of a high quality, various processes ranging from a simple flocculation and precipitation process to high grade processes utilizing membranes have been developed and actually used. As the apparatus used in such processes, for example, apparatuses using fine filtering membranes (MF membranes), ultrafine filtering membranes (UF membranes), reverse osmotic membranes (RO membranes) and dialytic membranes and electric deionizing apparatuses are known.
The electric deionizing apparatus, among these apparatuses, have a desalting compartment having a structure such that a cation exchange membrane and an anion exchange membrane are disposed as two side faces of the compartment and the space between the side faces is packed with an ion exchanger such as an ion exchange resin and an ion exchange fiber. Water is passed through the desalting compartment (differential pressure: 0.2 to 2 kg/cm2) while a voltage is applied to the compartment. Anions pass through the anion exchange membrane and are removed. Cations pass through the cation exchange membrane and are removed. The electric deionizing apparatus is characterized in that regeneration of chemicals is not necessary and that the apparatus is compact. Electric deionizing apparatuses having large capacities have been developed in recent years and are attracting attention.
As the semiconductor industry grows, demand for ultrapure water which is indispensable for the production of semiconductor devices is increasing. For the production of ultrapure water, the above electric deionizing apparatuses are increasingly used.
Ultrapure water can be produced by thoroughly removing all impurities which are not the pure water molecule and a complicated process is required for the production of ultrapure water. Processes having various combinations of steps are actually used. In basic, ultrapure water can be produced by conducting the following steps successively: (1) a preliminary step in which substances causing turbidity and colloidal substances contained in material water are removed; (2) a step in which fine particles, living organisms, salts and organic substances are removed with a reverse osmotic membrane; (3) a step in which dissolved gas molecules such as oxygen and carbon dioxide are removed by a degassing apparatus; (4) a step in which ions present in minute amounts are removed by an electric deionizing apparatus; and (5) the final step in which fragments from ion exchange resins, living organisms and other fine particles which are still present are completely removed by the ultrafine filtering membrane.
It is known that the deionizing treatment can be conducted in two steps by arranging two electric deionizing apparatuses described above in series and passing water through these apparatuses successively (Japanese Patent Application Publication No. Heisei 4(1992)-72567). It is also known that silica components are effectively removed by adjusting pH at 9.5 or greater in the electric dialysis (U.S. Pat. No. 4,298,442).
However, the process comprising arranging two electric deionizing apparatuses in series and conducting the deionizing treatment in two steps has the following problem with respect to the removal of silica components contained in material water.
When silica components are removed in the first electric deionizing apparatus, the amounts of ions other than the silica components such as sodium ion remaining in the deionized water decrease since these ions are more easily removed than the silica components. When the obtained deionized water is supplied to the second electric deionizing apparatus without any treatment and deionized, passage of the electric current through the water subjected to the deionization becomes difficult and the fraction of the removed silica components does not much increase in the second electric deionizing apparatus. In particular, when deionized water obtained in the first electric deionizing apparatus is used as water supplementing concentrated water in the second electric deionizing apparatus without any additional treatment, passage of the electric current in the second electric deionizing apparatus is extremely difficult and the fraction of the removed silica components is small.
The present invention has an object of providing an electric deionizing apparatus which has two electric deionizing apparatus arranged in series and effectively removes silica and boron components, in particular, silica components, contained in material water to provide deionized water having a high resistivity and a process for deionizing water having the above characteristics.
As the result of extensive studies by the present inventors to achieve the above object, it was found that an apparatus having two electric deionizing apparatuses arranged in series and a means for adding an aqueous solution of an electrolyte into a flow route of water which supplies water released from the first electric deionizing apparatus into the second electric deionizing apparatus is suitable for the purpose and that silica and boron components, in particular, silica components, are effectively removed and deionized water having a high resistivity can be obtained when material water is supplied to the first electric deionizing apparatus and deionized, an electrolyte is added to the deionized water and the obtained water is supplied to the second electric deionizing apparatus and deionized using the above electric deionizing apparatus. The present invention has been completed based on this knowledge.
The present invention provides:
(1) An electric deionizing apparatus which comprises a first electric deionizing apparatus and a second electric deionizing apparatus arranged in series, in which material water is deionized by the first electric deionizing apparatus and subsequently deionized by the second electric deionizing apparatus, and a means for adding an aqueous solution of an electrolyte into a flow route of water which supplies water released from the first electric deionizing apparatus into the second electric deionizing apparatus;
(2) An apparatus described in (1), wherein, as a flow route of water supplementing concentrated water in the second electric deionizing apparatus, a flow route branched from a flow route of water supplied to a desalting compartment of the second electric deionizing apparatus is connected to a flow route of the concentrated water;
(3) An apparatus described in any of (1) and (2), wherein a desalting compartment and a concentrating compartment of the second electric deionizing apparatus are each packed with an ion exchange resin;
(4) A process for electric deionization which comprises, using the electric deionizing apparatus described in any of (1), (2) and (3), supplying material water to a first electric deionizing apparatus, deionizing the material water by the first electric deionizing apparatus, adding an aqueous solution of an electrolyte to deionized water, supplying the deionized water to a second electric deionizing apparatus and deionizing the supplied deionized water by the second electric deionizing apparatus;
(5) A process described in (4), wherein the deionized water supplied to the second electric deionizing apparatus has an electric conductivity of 10 xcexcS/cm or greater;
(6) A process described in any of (4) and (5), wherein the aqueous solution of an electrolyte is an aqueous solution containing an alkali;
(7) A process described in (6), wherein the deionized water supplied to the second electric deionizing apparatus has a pH of 9.2 or greater;
(8) A process described in any of (4), (5), (6) and (7), wherein a concentration of silica in a concentrated water in the second electric deionizing apparatus is kept at 100 ppb or smaller;
(9) A process described in any of (4), (5), (6), (7) and (8), wherein desalting compartments in the first electric deionizing apparatus and in the second electric deionizing apparatus are operated under at least one of conditions of an applied voltage of 2 to 10 V/cell and an electric current efficiency of 10% or smaller; and
(10) A process described in any of (4), (5), (6), (7), (8) and (9), wherein the material water contains silica or boron.